There are numerous diseases and conditions that affect muscle. Examples include muscle wasting diseases, including cachexia, muscle attenuation or atrophy, including sarcopenia, ICU-induced weakness, surgery-induced weakness, neuromuscular diseases, and muscle degenerative diseases, such as muscular dystrophies.
Muscular dystrophy (MD) refers to a group of hereditary, progressive, degenerative disorders characterized by progressive muscle weakness, defects in muscle proteins, and the destruction of muscle fibers and tissue over time. In many cases, the histological picture shows variation in fiber size, muscle cell necrosis and regeneration, and often proliferation of connective and adipose tissue. The diseases primarily target the skeletal or voluntary muscles. However, muscles of the heart and other involuntary muscles are also affected in certain forms of muscular dystrophy.
There are several forms of muscular dystrophy, which differ in their age of onset, penetrance, severity, and pattern of muscles affected. Known forms of muscular dystrophy include Duchenne muscular dystrophy (DMD), Becker muscular dystrophy (BMD), Limb-Girdle muscular dystrophies, myotonic dystrophy (Steinert's disease), Emery-Dreifuss muscular dystrophy, Landouzy-Dejerine muscular dystrophy, facioscapulohumeral muscular dystrophy (FSH), von Graefe-Fuchs muscular dystrophy, oculopharyngeal muscular dystrophy (OPMD), distal muscular dystrophy, and congenital muscular dystrophies. While these are the main forms classified as muscular dystrophy, there are more than 100 diseases in total with similarities to muscular dystrophy. Some dystrophies may result from different underlying defects than others. Most types of MD are multi-system disorders with manifestations in body systems including the musculoskeletal, gastrointestinal and nervous systems, the heart, endocrine glands, skin, eyes and other organs.
Duchenne Muscular Dystrophy (DMD) is the most common inherited lethal childhood muscular dystrophy, affecting about 1 in 3000 males. Children with DMD usually become wheelchair bound by the age of 11 or 12 years and affected individuals usually die in the second or third decade of life. DMD originates from mutations in the dystrophin gene located on the X chromosome (Xp21), leading to loss of dystrophin protein with attendant muscle fiber destruction. Although the role of the dystrophin protein in maintaining skeletal myofiber integrity is generally well recognized, the exact mechanism that leads to myofiber destruction and loss in dystrophic muscle is not well understood. The discovery of the dystrophin gene and the subsequent characterization of the protein product have established dystrophin as an integral sarcolemmal protein, linking the muscle sarcomere and cytoskeleton to the surrounding extracellular matrix. The localization of dystrophin is synonymous with maintaining muscle integrity and its absence (as evidenced in DMD) leads to membrane fragility, contraction induced myofiber damage, and death (Petrof et al. 1993).
Becker type muscular dystrophy (BMD), also known as Benign pseudohypertrophic muscular dystrophy is an X-linked recessive inherited disorder characterized by slowly progressive muscle weakness of the legs and pelvis, which is also caused by mutations in the dystrophin gene, has onset in adolescence or adulthood with a less severe course of progression. BMD is related to Duchenne Muscular Dystrophy in that both result from a mutation in the dystrophin gene, but in DMD no functional dystrophin is produced making DMD much more severe than BMD. Both DMD and BMD have traditionally been called “X-linked” recessive diseases (Freund et al., 2007).
The limb girdle muscular dystrophies all show a similar distribution of muscle weakness, affecting both upper arms and legs. Many forms of limb girdle muscular dystrophy have been identified, showing different patterns of inheritance: autosomal recessive (designated LGMD1) or autosomal dominant (LGMD2). In an autosomal recessive pattern of inheritance, an individual receives two copies of the defective gene, one from each parent. In an autosomal dominant disease, the disorder can occur in either sex when an individual inherits a single defective gene from either parent. The recessive limb girdle muscular dystrophies are more frequent than the dominant forms, and may be more severe. Limb girdle muscular dystrophy can have a childhood onset, although more often symptoms appear in adolescence or young adulthood. The dominant limb girdle muscular dystrophies usually show adult onset. Some of the recessive forms have been associated with defects in proteins that make up the dystrophin-glycoprotein complex. Mutations in one component of the dystrophin-glycoprotein complex, the sarcoglycans, can lead to the forms of limb girdle muscular dystrophy known as LGMD2C, 2D, 2E, and 2F. Defects in caveolin 3, a protein that associates with the dystrophin-glycoprotein complex, lead to LGMD1C, while mutations in dysferlin, a protein that is thought to interact with caveolin 3, cause LGMD2B. Mutations in genes not related to the dystrophin-glycoprotein complex are implicated in other forms of limb girdle muscular dystrophy. For example, mutations in the enzymatic protein calpain 3 lead to LGMD2A (Guglieri M. et al., 2008).
Myotonic dystrophy is the most common form of muscular dystrophy. It is dominantly inherited and characterized by muscle hyperexcitability (myotonia), muscle wasting and weakness, cataracts, hypogonadism, cardiac conduction abnormalities and other developmental and degenerative manifestations frequently including cognitive dysfunction. Penetrance can be variable. Myotonic dystrophy can be caused by mutations in different genes, but the characteristics are quite similar. Type 1 myotonic dystrophy (DM1) is caused by expansion of a CTG triplet repeat in an untranslated region of the dystrophia myotonica protein kinase gene (DMPK) on chromosome 19, while type 2 (DM2) is caused by expansion of a CCTG repeat in the first intron of the zinc finger protein-9 gene (ZNF9) on chromosome 3. Repeat number in the myotonic dystrophies increases in subsequent generations (anticipation). DM1 also has congenital and childhood onset forms; these early appearing forms of the disease differ mechanistically from the adult form only in exhibiting larger CTG repeats that, in turn, trigger earlier appearance of symptoms. Those patients that survive early onset DM1 frequently exhibit morbidity and mortality in the third and fourth decades relating to cardiopulmonary involvement (Liguori C L. et al., 2001; Cho D H. et al., 2007).
Facioscapulohumeral muscular dystrophy (FSHD), a dominantly inherited disorder, is the third most common dystrophy after Duchenne and myotonic muscular dystrophy. FSHD is an autosomal dominant progressive degenerative disease that initially affects the muscles of the face (facio), shoulders (scapulo), and upper arms (humeral), followed by the muscles of the feet, pelvic girdle, and abdomen. Affected individuals may also suffer from hearing loss. Onset and progression of the disease is variable and often the weakness is asymmetrical in affected individuals. Life expectancy is typically within normal range, but the disease can lead to severe disability. Nearly all cases are associated with deletions of tandem repeats, termed D4Z4, in a distal region of chromosome 4 (4q35) (Tawil R., 2008).
The congenital muscular dystrophies are a heterogeneous class of disorders, and include several disorders with a range of symptoms. Muscle degeneration can be mild or severe, and may be restricted to skeletal muscle, or paired with effects on the brain and other organs. Defects in the protein merosin are responsible for about half of the cases in the U.S. Mutations in one of the integrin proteins gives rise to another form of congenital muscular dystrophy. Defects in the proteins called fukutin and fukutin-related protein cause the most common forms of congenital muscular dystrophy found in Japan. All of these proteins are thought to have some relationship to the dystrophin-glycoprotein complex. Some forms of congenital muscular dystrophy, including Fukuyama muscular dystrophy, muscle-eye brain disease, and Walker-Warburg syndrome are due to defective glycosylation of one of the proteins in the dystrophin-glycoprotein complex (alpha-dystroglycan) and show severe brain malformations, such as lissencephaly (a “cobblestone” appearance to part of the brain) and hydrocephalus (an excessive accumulation of fluid in the brain). Other forms, including the merosin-absent form and rigid spine syndrome, do not have major brain malformations associated with the disease. The molecular basis for many forms of congenital muscular dystrophy remains unknown (Sewry Calif., 2008).
Several other forms of muscular dystrophy also occur. Oculopharyngeal muscular dystrophy, which causes weakness in the eye, throat, and facial muscles, followed by pelvic and shoulder muscle weakness, has been attributed to a short triplet repeat expansion in the nuclear polyadenylate binding protein 1 gene (PABPN1), a gene involved in translating the genetic code into functional proteins. Inheritance follows either autosomal dominant or autosomal recessive patterns.
Polyalanine tract expansion from a norm of 10 to 12-17 residues causes aggregation of filamentous intranuclear inclusions in skeletal muscle which appear to precipitate the disease. This disease is most common in people of French-Canadian descent or people of Hispanic descent from certain regions of the Southwest. Miyoshi myopathy, one of the distal muscular dystrophies, causes initial weakness in the calf muscles, and is caused by defects in the protein dysferlin, which is the same gene responsible for LGMD2B, reinforcing the idea that progress against one form of muscular dystrophy should be informative to other forms. There are two forms of Emery-Dreifuss muscular dystrophy, an X-linked and an autosomal dominant form. Emery-Dreifuss muscular dystrophy is characterized by weakness in the shoulder girdle and lower legs, as well as the development of contractures in regions of the body, particularly the elbows, Achilles tendons, and neck. Defects in proteins that make up the nucleus, including emerin, and lamin A/C, are implicated in the disorder.
Several animal models, manifesting phenotypes observed in neuromuscular diseases, have been identified in nature or generated in laboratory. These models generally present physiological alterations observed in human patients and can be used as important tools for genetic, therapeutic, and histopathological studies. The study of animal models for genetic diseases, in spite of the existence of differences in some phenotypes, can provide important clues to the understanding of the pathogenesis of these disorders and are also very valuable for testing strategies for therapeutic approaches (Vainzof M, et al., 2008).
The mdx mouse model is a well-accepted animal model of human DMD. The mdx mouse carries a premature stop codon in exon 23 of the dystrophin gene and exhibits no detectable levels of dystrophin in muscle tissue. The progression of disease pathology in the dystrophic mdx mouse has been associated with constitutive activation of the MAP kinase, JNK1 (Kolodziejczyk et al. 2001), a ubiquitous signaling molecule. Once activated, JNK1 can phosphorylate the transcription factor NF-ATc1, leading to cytoplasmic accumulation and loss of NF-ATc1 function. Direct inhibition of JNK1 in dystrophic muscle, by overexpression of the JNK1 scaffolding protein JIP-1, was shown to reduce damage associated with typical disease progression (Kolodziejczyk et al. 2001). The present inventors have previously shown that the glucocorticoid, deflazacort, attenuates DMD pathology by circumventing and limiting the deleterious effects of JNK1 (St-Pierre et al. 2004). Deflazacort did not directly inhibit JNK1, rather the beneficial effects of this compound appear to originate from increasing the activity of the calcineurin phosphatase. Once activated, calcineurin then dephosphorylates NF-ATc1, restoring NF-ATc1 nuclear localization and transcriptional function (St-Pierre et al. 2004). Other groups have now demonstrated that increased calcineurin activity alleviates dystrophic muscle pathology (Chakkalakal et al. 2004; Chakkalakal et al. 2006; Stupka et al. 2006; Stupka et al. 2008).
A general interpretation of these studies is that calcineurin activation enhances myofiber integrity by increasing the expression of the dystrophin homologue utrophin, which itself provides an effective substitute for dystrophin in animal models of DMD. (St-Pierre et al. 2004; Chakkalakal et al. 2004; Chakkalakal et al. 2006). In agreement with this, enhanced utrophin expression has been shown to be an effective therapeutic intervention in a variety of dystrophy models (reviewed in Chakkalakal et al. 2005).
Currently, there are no cures for muscular dystrophy. Despite diligent research efforts to identify new therapeutic agents and new interventions for the treatment and management of MD, including of DMD, there has been limited success to date. Current treatments for DMD consist primarily of supportive care, including physical rehabilitation with braces, wheelchairs and ventilators, which can temporarily slow progression of disease and are essential in preventing complications and improving quality of life.
Corticosteroids (e.g., prednisone, prednisolone and deflazacort) are the only drugs that have been extensively studied as a pharmacologic therapy for DMD. However, controversies exist over their use because of the associated adverse effects, which include excessive weight gain, behavioral abnormalities, redistribution of body fat to the face and abdomen and away from the limbs, excessive hair growth, increased bone thinning and gastric ulceration, among others.
Prednisone is a synthetic corticosteroid drug that is usually taken orally, but can also be delivered by intramuscular injection. It is the corticosteroid most commonly prescribed for the treatment of DMD in North America. As with other steroid drugs, it is used to treat a number of different diseases and conditions. Prednisone is a prodrug that is converted by the liver into prednisolone, which is the active steroid. Prednisone can be effective in delaying the onset of symptoms of DMD, although the mechanism for the delay of symptoms is unknown.
Gene therapy offers future hope in the treatment of inherited single gene disorders, such as DMD, through targeting genetic defects and helping restore the defective protein. Indeed, it is widely believed that in the future, gene therapy could provide the cure for disorders such as DMD because it targets the disorder directly, whereas most other forms of treatment target only the symptoms of disease. However, at the present time, such therapy remains a distant reality and there is an immediate need for new and improved treatments.
It is, therefore, desirable to provide new compositions and methods for treating muscle diseases and conditions, including but not limited to, muscular dystrophy.